Introduction

Printed circuit boards (PCBs) are essential components of nearly all modern electronics. They provide the mechanical structure and electrical connections between components in an electronic device. PCBs are made from a variety of materials, but three of the most important are gold, silver and copper.

Gold, silver and copper each serve unique roles in PCB design and manufacturing due to their distinct mechanical, electrical and chemical properties. Understanding the use of these metals in PCBs can help designers optimize board performance, reliability and cost. This article provides an overview of gold, silver and copper in PCBs, including their properties, applications, advantages and limitations.

Gold in PCBs

Gold has become an indispensable material in the electronics industry due to its excellent conductivity, corrosion resistance and contact reliability. Here are some of the major uses of gold on printed circuit boards:

Wire Bonding

One of the most common uses of gold in electronics is wire bonding. This involves connecting integrated circuit (IC) chips to the underlying substrate or lead frame using thin gold wires. Gold is preferred for wire bonding because of its high ductility, malleability and electrical conductivity.

Fine gold wire with a diameter of 1 mil (25 μm) or less is bonded to the IC chip bond pads and substrate using thermosonic or thermocompression techniques. The resulting gold wire interconnects are highly reliable and allow electrical signals to be transmitted from the chip to the rest of the PCB.

Connector Plating

Connector contacts on PCBs are frequently plated with gold over nickel. The gold plating protects the nickel underlayer from corrosion and provides a highly conductive surface. This improves connectivity and prevents signal loss between mating connectors.

Gold-plated connectors also resist wear from repeated mating cycles. The gold surface remains smooth and retains its electrical characteristics better than bare nickel or tin plating. This makes gold ideal for high reliability applications.

Edge and Finger Plating

PCB edge and finger connectors are plated with gold over nickel to prevent corrosion and provide good solderability. The gold plating allows these connectors to be reliably soldered to a matching PCB socket or bus bar. This creates a durable mechanical and electrical interface.

Edge and finger connectors with gold plating withstand thousands of insertion cycles without signal degradation. The gold also protects the exposed nickel underlayer on the edges from oxidation.

Solder Pads

Many PCBs use gold plating on component solder pads. This allows components to be soldered to the board with consistent, high-quality connections. The gold readily dissolves into the solder, producing strong adhesion to both the component lead and pad. This prevents cracking or detachment during thermal cycling.

Gold immersion plating is commonly applied over nickel underplating on PCB pads. The thickness is typically 3 to 8 microinches (0.08 – 0.2 μm). This economical plating provides excellent solderability while minimizing the amount of expensive gold used.

Silver in PCBs

While gold excels at corrosion resistance and conductivity, its high cost mandates very selective application on circuit boards. For more cost-sensitive applications, silver provides an alternative with nearly equivalent conductivity. Common uses of silver in PCBs include:

Solder Pads and Traces

Immersion silver is an economical finish for many solder pads and traces on PCBs. It eliminates the need for an additional solder coating while providing good shelf life. The dissolution of silver into lead-free solders improves wetting and joint strength.

Silver plating withstands multiple reflow cycles, though some care is needed. Gold remains the premium choice for the utmost reliability in extreme conditions.

Wire Bonding

Some wire bonding applications utilize silver instead of gold wire. Silver has around 7% lower conductivity than gold, but costs much less. Fine silver wire down to 1 mil (25 μm) diameter can be wedge or ball bonded to chips and substrates.

This lower cost option is acceptable for many consumer electronics and other cost-sensitive wire bonding uses. However, gold is still preferred for high-reliability wire bonds.

RF Applications

Silver offers the best electrical performance after gold for radio frequency (RF) applications. It has the highest conductivity of any metal except gold and copper. This makes it suitable for use in RF transmission lines and circuit traces.

Silver plating is also used on connectors and components for microwave and millimeter wave circuits. It provides lower insertion loss compared to other surface finishes.

EMI Shielding

Silver’s excellent conductivity gives it advantages for electromagnetic interference (EMI) shielding. Silver-plated plastic and metal housings can be used to contain and block EMI from electronics. The high conductivity creates an effective Faraday cage to attenuate radiated emissions.

Thermal Management

Silver-filled epoxy adhesives provide enhanced thermal conductivity while bonding components. This improves transfer of heat from hot components to heat spreaders or heat sinks. The thermal performance is not as high as solders, but the adhesives avoid soldering temperature restrictions.

Copper in PCBs

Copper is the standard base metal used to form conductive traces and pads on most PCBs. Here are some key roles of copper in PCB design and fabrication:

Circuit Traces

The vast majority of conductive traces on PCBs are composed of copper. Copper has the second highest conductivity after silver and offers the optimum balance of performance and cost. Circuit traces are formed by selectively etching away unwanted copper from a clad laminate base.

Trace widths and clearances down to 5 mils (0.127 mm) or less enable complex circuity with fine feature sizes. This allows high component densities on the PCB.

Planes and Shields

Large copper regions on inner or outer layers can provide power or ground planes over most of the board area. This gives a low impedance reference and quiet power distribution network. The planes also shield circuits from electromagnetic interference.

Additional copper planes or shields can be strategically placed within the board stackup to isolate critical signals. Careful partitioning of the copper layers controls crosstalk and EMI.

Heat Dissipation

The excellent thermal conductivity of copper makes it ideal for heat dissipation in PCBs. Copper planes act as heat spreaders to conduct heat away from hot components. Copper also allows vias and thermal pads to efficiently conduct heat to inner layers.

High power circuits benefit from bottom side copper planes to spread heat. Additional thermal vias help transfer heat to bottom layers.

Component Leads

Most through-hole PCB components have leads made of copper alloy due to its good balance of strength and conductivity. The leads can be reliably soldered to copper PCB pads and through-plated holes.

Surface mount components often use copper leadframes which are soldered to the copper pads on outer layers. This provides strong electrical and mechanical joints.

Interconnects

In addition to standard copper traces, PCBs use copper for various interconnect technologies. Copper foil sheets in multilayer boards interconnect outer layers through drilled holes. Buried and blind vias allow connections to inner layers. Direct copper bonding allows mounting of power devices.

PCB Surface Finishes

While copper, silver and gold serve core functions in PCBs, surface finishes are equally critical. Bare copper and metals quickly oxidize and degrade solderability. Many options exist for protecting pads and traces:

Organic Solderability Preservatives (OSP)

OSP coatings are organic compounds that shield copper surfaces from oxidation. They provide several months of shelf life before soldering with good wetting. OSP is affordable and common but offers no corrosion protection after soldering.

Immersion Tin

Immersing PCBs in molten tin results in thin but uniform plating. The tin finish quickly alloys with solders for reliable wetting and joints. However, tin eventually grows conductive whiskers that risk short circuits.

Immersion Silver

Silver coatings applied by chemical reduction provide excellent shelf life and solderability. Silver also hardens lead-free solder joints. It tarnishes slightly over time but has better long term reliability than OSP or tin.

Electroless Nickel Immersion Gold (ENIG)

Combining electroless nickel and immersion gold provides the best of both metals – nickel for corrosion protection under gold. This finish offers the longest shelf life and can withstand multiple reflow cycles. It is more costly than OSP or silver but provides excellent reliability.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

To further enhance corrosion resistance and solderability, a palladium layer can be added between the nickel and gold in ENIG plating. This reduces gold thickness for cost savings while achieving similar performance.

Metal Foils for Flexible Circuits

Flexible printed circuits (FPC) use different construction than rigid PCBs. Polyimide substrates are clad with ultrathin copper foil rolled to 5 to 35 μm thickness. FPCs can also use alternative metal foils:

Stainless Steel

Plating copper and etch resistant layers onto stainless steel foil allows fabricating stainless steel flex circuits. These robust, chemical resistant circuits operate in hash environments up to 800°F. Stainless steel has worse conductivity than copper but suits niche applications.

Aluminum

Aluminum exhibits good flexibility and thermal conductivity in thin foils under 3 mils (76 μm). Aluminum FPCs are lower cost than copper but suffer from oxidation and poor solderability. Interface pads plated with nickel and gold are required for connections.

Beryllium Copper

Beryllium copper (BeCu) is another flexible foil option. It has high strength over a wide temperature range coupled with good conductivity. BeCu flex circuits suit dynamic applications with continuous flexing. The foil is typically 7 to 140 μm thick.

Key Design Factors for Metals

Some key considerations when selecting metal materials and finishes in PCB design:

• Operating temperature range – Some metals like aluminum or silver perform poorly at elevated temperatures. Gold and high-temp solders are needed for hot environments.
• Corrosion resistance – Silver and tin corrode quickly when exposed to humidity or contaminants. Gold, nickel and palladium provide excellent protection.
• Conductivity and impedance – High frequency, EMI shielding and power distribution all benefit from gold, silver or copper instead of less conductive metals.
• Current density – Copper and gold tracks can reliably carry higher current loads than aluminum or steel before electromigration damage.
• Thermal loads – Copper and silver offer the best thermal conductivity to dissipate heat from high power PCB components.
• Environmental exposure – Harsh indoor or outdoor environments demand gold plating or stainless steel circuits rather than plain copper.
• Cost – Expensive metals like gold must be used selectively for cost-effective PCB design. Standard copper with organic coating can suffice for benign environments.
• Soldering methods – The required soldering and assembly process influences suitable PCB surface metals and platings.
• Rework needs – Metals and platings like ENIG allow repeated rework and soldering with consistent performance.

With careful selection, the combination of copper, silver and gold in PCB fabrication provides an optimal balance of cost, reliability and performance across a wide range of electronic products.

Frequently Asked Questions

Question 1: Why is gold used for coating connector fingers when they mate with tin-plated sockets?

Answer: Gold is used to coat connector fingers because of its corrosion resistance and compatibility with tin. Bare copper would oxidize and degrade after repeated mating cycles with tin-plated connectors. This oxidation could cause unstable contact resistance. Gold avoids this issue and also prevents fretting corrosion. The dissimilar metals prevent galvanic corrosion even with minor rubbing between the contacts. So gold plating maintains low, stable contact resistance despite thousands of mating cycles with tin-plated sockets.

Question 2: How are the fine gold bonding wires attached to IC chips in wire bonding?

Answer: Fine gold wire down to 1 mil diameter is bonded to IC chips using either ultrasonic or thermosonic techniques. With ultrasonic bonding, the free air ball at the end of the wire is pressed against the chip bond pad while high frequency (60-120 kHz) ultrasonic energy is applied through the bonding tool. This fuses the ball to the pad. For thermosonic bonding, heat around 150°C is added simultaneously with ultrasonic energy to further assist bonding and require less force. The ultrasonics create localized heating, pressure and scrubbing action that fuse the gold ball to the bond pad without damage. Similar techniques are used to attach the other end of the wire to the substrate.

Question 3: Why does immersion silver sometimes cause charring of FR-4 PCB laminates during soldering?

Answer: Immersion silver can occasionally cause charring or deterioration of FR-4 resin during soldering. This is due to galvanic corrosion between the silver and copper traces in the presence of solder flux residues after incomplete cleaning. The flux residues contain ionic contaminants that create localized galvanic cells when heated above 130°C. Aggressive attack of the resin occurs at the copper-silver junction. Use of “no clean” flux formulations with low ionic activity helps prevent this problem. Improved cleaning to remove all flux residues also prevents charring.

Question 4: How is direct copper bonding used to mount power devices on PCBs?

Answer: Direct copper bonding uses a thin layer of copper foil to bond power devices like IGBTs or MOSFETs to an aluminum nitride or alumina ceramic substrate. First, an adhesion layer like titanium or chromium is sputtered onto the ceramic. Next, pure copper foil 1-3 mils thick is oxidized at elevated temperature so it bonds strongly with the ceramic. Devices can then be brazed or soldered to the copper. The direct copper bond provides excellent thermal conductivity for heat dissipation while the ceramic insulates the exposed bottom of the devices.

Question 5: Why are flexible PCBs sometimes fabricated with beryllium copper foil instead of standard rolled copper?

Answer: Beryllium copper foil is used when the flex circuit will undergo dynamic flexing rather than just static bending. The beryllium oxide particles in BeCu increase strength and spring properties without compromising conductivity. So BeCu flex circuits maintain good electrical performance after repeated bending cycles. This makes beryllium copper a good choice for dynamic FPC applications like print heads or exercise equipment. The alternative copper alloy C7025 with 25% nickel also provides enhanced flexure life.